Lecture 5 Flashcards
When you have lots of prey and only one predator it is difficult for them to focus on one individual so what increases an individuals chance of being predated?
. Anything that will make them stand out
. If you stray from the group
What animal was used to experiment saliency model? How did they carry the experiment out?
. Sticklebacks (he thinks)
. They dyed one a different colour so it would stand out so the predator will go for it
. Also, individuals that stray from the shoal will get picked off because it is easier
What do saliency models tell you?
Tell you what your eye will automatically be drawn to, so they tell you what will ‘pop out’.
What is isogamy/ anisogamy?
Is the evolution of the gametes because they are the sex cells that are the fundamental difference between the males and females
What is isogamy?
Is when you have equal sized gametes meaning neither male or female forms
What organisms usually/ many of them have isogamy? Give an example of an organism that this appears in
This occurs in many simple unicellular organisms.
E.g. Paramecium caudatum
(In almost all multicellular organisms that reproduce sexually the gametes are not equal sizes)
What is anisogamous?
Is when one of the gametes (the egg) is large, immobile and food rich and the other gamete is small, mobile and is essentially just a delivery mechanism for DNA
Why is anisogamy important?
As it is this discrepancy in gamete size that is a major driving factor in sexual selection, we need to understand this.
The food reserves become significant so you get the evolution of anisogamy- it is more common as zygotes require more food reserves to grow
Why does the survival of a zygote depend on its size?
Because the bigger the zygote the greater the survival chances as it has more resources available, but this is a weigh up because of you have larger gametes then you have fewer.
So, less gametes could be produced at a larger size so the survival benefit must be great enough to compensate for this
Once you have larger gametes how does it affect selection pressure and why?
Because once you have these larger gametes there is immediate selection pressure for smaller gametes because these smaller gametes will seek out and parasitise the larger gametes food reserves, because they do not have as much of a food supply
How do larger gametes respond to smaller gametes trying to parasitise them and why do they respond this way? What is the outcome?
The larger gamete will try to resist this as the greatest chance of survival would be from the fusion of two large gametes.
However, due to the vast numbers of smaller gametes the small-large fusion will still predominate, so if you have a lot of smaller gametes trying to parasitise a large gamete than they will eventually win. Large gametes will suffer greatly when fussing with a small gamete. So all of this means that selection will act much more strongly on smaller gametes
What does the greater numbers of smaller gametes mean for the smaller gametes?
Means increased genetic diversity as well as vastly increased mortality and the strength of selection is directly proportional to the amount of genetic density, so the greater the genetic diversity the greater the strength of natural selection will be, there will be more mortality as well.
This increased selection pressure will enable the smaller gametes to overcome the defences of the larger gametes
What will the arms race between smaller and larger gametes eventually lead to?
Individuals specialising in producing few large gametes (females) or many small ones (males)
Isogamy is prevalent in single cell forms, why is this?
Because food reserves for the zygote are much less important (so there are very few food reserves). So they tend to be exactly the same size because there are no food reserves
Describe male in terms of reproductive success
Best increase success by finding and fertilising many different females
Describe in terms of female reproductive success
Best increase reproductive success by increasing the rate of converting resources into egg and offspring
Once they mate and get their eggs fertilised if they mate anymore it doesn’t make anymore contribution to their reproductive success
They are predicted to be a bit more choosy
What is intrasexual selection? What did Darwin suggest this would drive?
Competition between members of the same sex (usually males).
He suggested that this would drive the evolution of increased size or weapons such as horns
Give examples of species that have been influenced by intrasexual selection
. Bighorn sheep (Ovis canadensis)
. Red deer (Cervus elaphus)
. Elephant seals
(. Also, sexual dimorphism because the bigger a male is the more successful in a fight)
Explain the example of where intraspecific selection has influenced elephant seals/ how
. Female northern elephant seals haul up on beaches to give birth and then mate again
. These large groups provide males with a defendable resource
. The largest, strongest males can claim the largest harem
. So, there is a strong selection for size because if you are larger then you can defend these harems better and fend off other males better so you get a lot of sexual dimorphism
When Le Boeuf and Reuter (1988) did a long term study on mating elephant seals. What did they find?
Found that 5 of 180 males were responsible for 48-92% of mating’s with 470 females
Why do males not try and breed until between 6-9years?
Because they need to build size and strength for their success is down to their ability to fight rivals
At what age do female elephant seals start to breed? And what is their success down to?
4 years. Female success is down to ability to nurture young
Describe intersexual selection. What should it do?
. Competition between (usually) males but the difference is that the females choose the winner
. Should drive the evolution of extravagant features such as extravagant plumage or colouration, territory defence and song (so selection for behaviour not just physical traits)
In what species do you see intersexual selection? (Give specific examples as well)
Seen in huge numbers of birds and fish such as peacocks (plumage)
Wrens (territory)
Goldfinch (song and body colouration)
Recent studies have also suggested that male mice sing to their mates
Explain the study by Andersson (1982) looking at intersexual selection in long-tailed widowbird (how they did it)
. Female long-tailed widowbirds (Euplectes progne) females select mates based on tail length (up to 50cm)
. He first mapped 36 male territories
. Number of females attracted (nests) was recorded
. They were then divided into 4 groups
. The first group had their tails cut to ~14cm
. The second group had these severed tail pieces superglued to their tails to artificially elongate them
. 2 control groups: 1 was unaltered, 1 had tails cut then glued back on (because they thought there might be discontinuity because they had to glue the tail back on)
Sedge warbler males sing until they pair up (monogamous) what was shown in the field that reduced the time it took for a male to pair up? Why is this?
The number of different repertoires/ number of songs the male knows. Males that have a very large number pair up with females much earlier and females respond much more strongly to males that have a large number of songs as well.
So males with the most elaborate songs pair up more quickly.
This is because sedge warblers with larger repertoires make better parents so females get this advantage
Give examples of species that control territories, how do females benefit from these males?
. Widowbirds
. Sedge warblers
Control territories so access to good nest sites, so this is a resource that females get from choosing the correct males
What do females get from being choosy about males?
. Genetic benefit so assess the quality of males
. In some species females get some resources from it
Give an example of a species where advantages of mate choice to females include gifts of food. How does it work in this species?
The hanging fly.
When a male is looking for a mate it hangs off a branch or leaf and has a fly or something that it offers to the female during mating. Has been shown that the size of the prey is linked to the length of copulation. So, the larger the gift the better access to females
Give an example of a species where females benefit some mate choice do to the transfer of toxins. How does it work?
The Bella moth.
During mating males transfer substances that predators find unpleasant so females pick a good mate so they get more of these and it helps protect them from predators
Give an example of a species where the male provides no parental care
Bower birds
How do male bower birds attract mates?
. No parental care
. All reproductive effort is put into display- use colourful items to decorate bowers: berries, beetles, stones
. Manmade items too: pegs, bottle tops etc.
Give an example of a bowerbird species that has been studied in mate attraction and what they found
Satin bowerbird.
. Multiple studies have shown that the more elaborate bowers attract the most female attention
. Removal of decorations experimentally reduces mating success
. The males also steal from each other and even destroyed other males bowers (competition)
Madden (2002) experimented male competition/ mate attraction in bower birds. What was found?
. Added berries to bowers of spotted bowerbirds (the males also dance around a bit as well so that is a behavioural change)
. This increased attack’s from other males
Vogelkop bowerbird bowers were studied as to how they decorated their bowers. What did they find? What does this suggest?
When berries were removed and birds were given excess berries to redecorate males only restored the previous number of decorations, so they built it exactly the same size and did not build it bigger than it was before
When the experiment was carried out on Vogelkop bowerbird bowers what did the results suggest? So how is the female making her choice?
That bower decoration is an indicator of social status giving information about the quality of the male. So, if a bird that has a low social status builds a bigger and flashier bower then the other males will come and knock that down.
Although the female gets only sperm from the male she is still making a choice based on the available information. Likely a genetic benefit as these males are likely to be a higher social status in the community. (Females cannot test for this therefore must look for indicators of quality
What is fishers hypothesis (also known as the ‘sexy son’ hypothesis)?
. Fisher suggested that females find certain traits attractive
. Therefore, by mating with these attractive males females will have attractive offspring
. Suggests that females have eyes that are stimulated by a certain trait
Give an example of how ‘sexy sons’/ fisher hypothesis could evolve
. A long tail could improve flight ability giving an advantage in foraging or predator avoidance
. Image that a female just randomly starts liking a male with a longer tail, if it mates with that male then the young females will have a preference for a longer tail and the males will have long tails as well. So, you start to get genetic associations on chromosomes. So, you start to get these genetic associations between individuals for pronounced features and female preference for it. These connections between male traits and female preference genetically can just become more and more pronounced through time and you get this runaway preference (positive feedback loop)
. Or they could simply have been easier to detect in a complex environment
When does the runaway selection/ the positive feedback loop stop?
When the trait becomes too much of a handicap then selection will cease pushing it in that direction due to decreases in survival
What is the handicap hypothesis?
Zahavi (1975) noted that extravagant male features actually create a handicap. He suggested that this handicap was the reason females chose these males as they are good indicators of male quality.
A male must be very good at surviving to do so with a handicap (because extra weight, more visible to predators so would have to be a high quality male to survive). So this increased survivability is heritable and will be passed to offspring
What must be true for the handicap hypothesis to work?
The males must only express the handicap when in good condition.
Must be a flexible trait so has to be like when birds have patches under their wings that they can show when they want to.
Suggest problem was that this process would very quickly exhaust male genetic variation traits if females are only choosing these males that have these traits so very soon the genetic variation for anything different is removed from the population
(Goldie’s bird of paradise)
At first the handicap hypothesis was not accepted because it was suggested that genetic variation would be exhausted very quickly. However, subsequent work has provided evidence for this idea in 4 ways. What are the reasons why genetic variation won’t be exhausted?
1) populations constantly gain mutations, often deleterious ones. It payed to remain choosy
2) lots of different genes contributing to the male condition so it is unlikely to reduce genetic variation (because there is not only one gene causing it) as there are many different factors contributing to male condition
3) females can choose different traits each year
4) display is a good indicator of resistance to disease/ parasites: good genes hypothesis. And because parasites and prey are evolving very quickly together this is another thing that is thought to maintain this genetic variation
What was the suggestion Hamilton and Zuk (1982) made that had been tested experimentally?
. They suggested that sexual displays are good indicators of resistance to disease or parasites
. As diseases and parasites are in a constant arms race with hosts this will maintain choosiness in females as the genetic variation involved will be rapidly evolving
What animal is often used in studies of speciation and why?
. Cichlid fish
. Due to their variety of species and geographical proximity to each other
How can behavioural isolation occur?
Females selecting males with exaggerated characteristics can cause very rapid evolution. If this selection becomes strong enough it can cause behavioural isolation. So conventional sexual selected traits; courtship display differences, preference for more extreme traits etc. That combined with conventional speciation mechanisms can really move it along
Give an example of behavioural isolation
Hawaiian Drosophila:
. Males of Drosophila silvestris have narrow heads
. Males of Drosophila heteroneura have wide hands
. Found on different islands
. Found no different I’m female choice for head size
. However, differences in mating behaviour did reduce mating between species/ different traits that will stop gene flow between them
. Timing of the mating display differs between species and this prevented mating from occurring
Give and example of sympatric speciation occurring
. Several studies of cichlid fish species
. Many species living in close proximity
. Capable of interbreeding
. They do not interbreed due to female preferences for male characteristics
. Was concluded from a large dataset that sympatric speciation was taking place
What are the two factors driving sympatric speciation in cichlid fish species?
1) female preference for male colour
2) competition for nest sites between males
What is sympatric speciation in cichlid fish species an example of?
Negative frequency dependence that can maintain polymorphism
Why is sympatric speciation of cichlid fish species an example of negative frequency dependence that can maintain polymorphism?
. The males look differ
. If a male comes to a nest a male is guarding that is a bit different (different colour) they won’t compete
. But if one comes that is the same colour then they will compete
. So, this provides an advantage for polymorphisms
Why is there thought to be a bit of ecological selection in cichlid fish species?
They think that their mouth parts differ a bit and hybrids are not able to handle food substances.
What is allopathic speciation?
When you have two completely isolated populations then they will diverge in completely different directions through sexual selection. Then when they come back together they can be completely speculated, so won’t mate with each other
What can be a driving force for allopathic speciation?
Sexual selection
Explain how in polygamous species sexual selection can cause allopatric speciation. And what will increase the effect?
. In polygamous species males invest very little in mating and will attempt to manipulate females to provide the reproductive effort required
. To maximise fitness, they must also outcompete other males even at the expense of females
. Females will evolve to counter these costs
. This has the potential to drive rapid evolutionary change, especially in allopatric populations
. Model by Gavrilets suggests that this is feasible and that the effect will be increased with population size
What is postzygotic speciation?
The inability to produce fertile offspring
When you have had two populations that have diverged what often happens when you bring them back together? What ‘rule’ is this?
On of the sexes is rarer and that tends to the heterozygous.
So they are sterile when they come back together- often males.
Haldane’s rule
If in allopatric speciation you diverge apart for a million years and then come back together and hybrids will still fit what will happen?
That will really be the end of the speciation process, genes will be exchanged between these different populations and speciation will disappear
What does the ‘faster-male’ theory try to explain?
Why postzygotic isolation is more common in males. It tries to explain this through sexual selection, so the idea is that the male sterility is more common than the female sterility due to spermatogenesis being more sensitive to evolutionary change
Other than the ‘faster-male’ theory what is another explanation for why postzygotic isolation is more common in males? What is a problem with this?
Could be sexual selection or conflict causing increased divergence in male genetics.
The problem with this is heterogametic females also tend to be sterile, so it is not just males.
However, evidence shows that it could still be true
The association between a females preference and a male feature characteristic such as tail length is called what?
Covariance
What is hybrid fitness a big thing in?
Allopathic speciation and sympatric speciation
Give examples of when haldane’s rule has been observed
Seen in 151 out of 157 crosses in Drosophila and mammals (male heterogametic) and 91 out of 98 crosses in Lepidoptera and birds (female heterogametic)
